1. Field of the Invention
Embodiments of the present invention generally relate to optical interrogation and, more particularly, to techniques and apparatus for accurately determining a characteristic wavelength of an optical component when using swept wavelengths.
2. Description of the Related Art
Many optical components have a characteristic wavelength that may be found by interrogating the optical component with an optical source capable of producing light at various wavelengths over a fixed range or bandwidth. For example, Bragg gratings (typically formed by photo-induced periodic modulation of the refractive index of an optical waveguide core) are highly reflective to light having wavelengths within a narrow bandwidth centered at a wavelength generally referred to as the Bragg wavelength. Because light having wavelengths outside this narrow bandwidth is passed without reflection, Bragg wavelengths can be determined by interrogating a Bragg grating with a light source swept across a bandwidth that includes the Bragg wavelength and monitoring the reflected optical power spectrum at a receiver unit. Because Bragg wavelengths are dependent on physical parameters, such as temperature and strain, Bragg gratings can be utilized in optical sensor systems to measure such parameters.
In these and a wide range of other types of optical systems, the measurement of a characteristic wavelength of an optical component to great accuracy (and/or with great repeatability) is important to system performance. For some techniques, the measurement of characteristic wavelength may be determined using a wavelength sweep technique, where light introduced into the system (e.g., into an optical waveguide) is varied across a range of wavelengths. The introduced light is transmitted down the waveguide and a portion of this light may be reflected back by a Bragg grating if the wavelength of the transmitted light matches the Bragg grating's characteristic wavelength. If the rate of the sweep, the length of the waveguide between the optical source and the Bragg grating, and the index of refraction of the waveguide's core are known, the characteristic wavelength of the reflected light, and thus of the Bragg grating, may be determined based on a time at which a reflection peak is received (i.e., the round-trip time for light introduced into the system to interrogate the Bragg gratings and return to a receiver unit). However, light traveling in a waveguide experiences a time delay depending on the length and the index of refraction of the waveguide. Accurate determination of the time delay can increase characteristic wavelength measurement repeatability and accuracy.
Accordingly, techniques and apparatus for accurately determining the time delay of reflected signals in an interrogation system are desirable.